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Presented at the NABS Annual meeting, Pittsburgh, Pennsylvania, 2002
in Food Webs
BIOMASS VERSUS ABUNDANCE: MEASURING THE IMPORTANCE OF PREY TYPES FROM GUT ANALYSES.
N.E. Leonard. Department of Biological Sciences, University of New Orleans, New Orleans, LA 70148
The importance of benthic prey items for vertebrate predators is often determined using tallies of prey abundance from gut contents, with the assumption that abundance and biomass of prey items are highly correlated. Here I test this assumption using counts and biomass of prey items collected from the stomachs of 50 Ascaphus truei from Mack Creek in Oregon. Prey items were categorized according to a coarse classification scheme (i.e., foraging habitat). The data were analyzed using Canonical Correlation Analysis to detect relationships between the abundance and biomass variables. The strongest correlation between the canonical variables was r = 0.6610 and primarily related spider biomass to spider abundance. Similarly, abundance and biomass of the emergent aquatic invertebrates canonical variables was correlated at r = 0.6072. Low levels of correlation between abundance and biomass canons indicate that tallies of ingested prey items are not suitable surrogates for prey biomass when using a coarse classification scheme. For example, when using abundance of prey items, emergent aquatic invertebrates appear more important than spiders, but biomass data yield a different conclusion. Thus, when addressing the importance of prey types or foraging habitats for predators, abundance should not be used as a surrogate for biomass.
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